The Green Flash
It has been a good day today. The weather is good and the ROV has been down since lunch sending back some beautiful images from below. We will have a busy night of processing samples when the ROV returns to the ship in an hour or so. After being in the lab for most of the day I realized that I have been outside less on this trip than I have been in quite some time, so I headed out to the bow for sunset. And I’m glad I did. There were a few clouds, which always makes sunsets more interesting, and the sky was very clear (no haze). I photographed the setting sun and then saw one of the NOAA Corps crew bring a pair of binoculars to one of the science crew and I thought…Green Flash! I have not seen one in many years, but this was a perfect opportunity. I was taking pictures through my wide angle and my 300 telephoto lenses, and at the last moment just before the sun sank below the horizon, there it was: flashes of green on the setting sun through the telephoto lens. The photos here will not do it justice, even though I have greatly cropped the last image to attempt to show you some green color.
What I witnessed is an optical phenomenon that can occur shortly after sunset or before sunrise, when a green spot(s) is visible, usually for no more than a second or two, above the setting sun. They are supposedly best seen with unobstructed horizons like you have here on the ocean.
The reason for a green flash is a bit complicated but basically has to do with refraction of light (like in a prism). Light moves more slowly in the lower, denser air than in the thinner air above, so sunlight rays follow paths that curve slightly, in the same direction as the curvature of the Earth. Higher frequency light (green/blue) curves more than lower frequency light (red/orange), so the green rays of the upper portion of the setting sun remain visible a tiny bit longer after the red/orange rays are obstructed by the Earth.
But whatever the reason, it was an incredible sight, and you can bet I’ll be back up on deck for future sunrises and sunsets.